Currently, it is believed that IC tags are products in the entrance to ubiquitous era. The IC tags have been developed as RF-IDs (ultra-compact radio frequency identifications) in, for example, name tags, Suica cards, and FeRAM cards. Many people expect that the IC tag market will certainly grow in the future. However, the market has not been expanded as expected. This is caused by problems that should be socially solved, such as cost, security, and confidentiality.
The RF-ID technologies may also be applied for identifying documents having proprietary values, such as bank notes and securities. It may be possible to solve the problems such as forged bank notes by embedding IC tags in bank notes or the like. However, it has not been realized yet because the IC tag is expensive and has a large size.
The price of the IC tag can be decreased by reducing the size of its IC tag chip. This is because that a reduction in size of the IC tag chip allows producing a large number of IC tag chips from one wafer. An IC tag chip with a size of 0.4 mm square has been developed at the present time. This IC tag chip allows reading out 128-bit memory data in the chip with a microwave of 2.45 GHz (for example, see Non-Patent Document 1).
Separately, a radio frequency automatic identification (RF/AID) system that uses an element other than IC tags and can be applied to, for example, identification of bank notes and credit cards has been also investigated. As an example of such a system, in Patent Document 1, a plurality of resonators that resonates with a plurality of radio frequencies is fixed on a substrate of paper or plastic such that they occupy random spatial positions on the substrate. The plurality of resonators is a passive solid resonator. The passive solid resonator includes a thin dipole made of an extended metal. More specifically, the passive solid resonator is made of a material belonging to a quartz family, such as quartz crystal. In a radio frequency target, a plurality of resonators disposed on a substrate resonate when they are each irradiated with an electromagnetic wave of radio frequency, and the alignment of the plurality of resonators is comprehended and identified by detecting the resonance.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 10-171951
[Non-Patent Document 1] Mitsuo USAMI, “An ultra small radio IC tag chip: μ-chip”, OYO BUTURI (Applied Physics), Vol. 73, No. 9, 2004, pp. 1179-1183.